Explain Opaque Types

[JoshOrndorff]

A Pattern in Substrate is that there are some types in the runtime which have corresponding opaque types in the outer node. Opaque means that the outer node cannot inspect the data contained in hte type or use other type system features like pattern matching. Rather the outer node see the data only as Vec. This allows the outer node to abstract over these types, and not need upgrading when the runtime is upgraded and the types change.

This is done at least for the Extrinsic and Block types.

@bkchr (because you requested this on riot) Can you confirm whether my limited understanding above is correct, and provide a few examples of types that follow this pattern?